Adverse outcomes in cardiac surgery associated with cardiopulmonary bypass (CPB) include neurocognitive deficits, myocardial ischemia, peri-operative bleeding, and post-operative thrombosis. Our overall goal is to elucidate the underlying pathophysiology of CPB which involves activation of, and cross-talk between, cellular and soluble hemostatic and inflammatory systems, with the aim of devising therapeutic interventions. Our studies are based on both in vitro models and clinical material. Under the past aegis of this grant, we have helped define normal physiology of platelet-leukocyte interactions; shown that CPB results in upregulation of specific p 2 integrins on leukocytes, P-selectin on platelets, and formation of circulating leukocyte-platelet conjugates in vitro and in vivo; defined differential roles for specific complement and coagulation activation products in the induction of platelet versus neutrophil versus monocyte activation; shown that C5 complement blockade carried out in clinical CPB is associated with similar effects to those seen in vitro, and, in a pilot study, a reduction in CPB adverse outcomes; and discovered that the PL-A1/A2 genetic polymorphism of platelet gp llla is associated with different neurologic and myocardial outcomes in clinical CPB. Our new specific aims are: (la) using in vitro models, determine the role of the C5a and C3a receptors in neutrophil, monocyte and platelet activation induced by simulated extracorporeal circulation (SECC); since preliminary data suggests the unexpected finding that C5aR blockade abrogates platelet activation, determine the responsible mechanism; (1 b) determine the role of the lectin complement pathway in CPB by (i) correlating in vivo complement activation with interpatient genetic variability in mannose binding lectin (MBL) levels and acquired CRP levels, (ii) examining the role of MBL in SECC; (2a) determine differences in human monocyte subsets, defined by CD64/CD16/CD14 expression, with respect to p.2 in and tissue factor regulation; (2b) examine monocyte subset distribution during in vivo and in vitro CPB; using expression array technology, define the pattern of monocyte inflammatory gene expression induced by SECC; (3) examine the effects of SECC on human endothelial cells by in vitro modeling, specifically including alterations in tissue factor regulation and the effect of differential complement components